1. Field of the Invention
The present invention relates to residential and commercial water treatment systems, and, more particularly, to pressurized tank liners, fittings and coupling devices for residential and commercial water treatment systems.
2. Description of the Related Art
Residential and commercial water treatment tanks are generally used to hold either a sacrificial or filtration media through which water is directed for treatment. For example, the removal of iron, hydrogen sulfite, copper, zinc, lead and other compounds often requires adjustment of the water's pH. A common method of adjusting pH is by the use of calcium carbonate as a sacrificial media. This requires the periodic replenishment of the sacrificial medium placed within the water treatment tank. It is desirable to have a hole in the dome or top of the tank to ease this replenishment. Filter media, such as activated carbon, filter sand, anthracite coal and various grades of gravel, are used to remove unwanted taste, odor, color and turbidity. Periodic access is also necessary for such filter media to either replace or clean the media.
Early water treatment pressure vessels were welded steel tanks which were both heavy and susceptible to corrosion even when painted and epoxy lined. With the development of fiberglass reinforced plastic tanks (i.e., FRP tanks) corrosion problems were eliminated. FRP tanks, however, were susceptible to impact damage which often occurred during transport. Although FRP tanks are capable of withstanding high internal pressures, impact damaged areas inevitably leak and cannot be repaired. Filament wound tanks eventually replaced FRP tanks. Early filament wound tanks, although having a rugged exterior, had a brittle ABS (i.e., acrylonitrile butadiene styrene) plastic liner which was susceptible to cracking. Currently, the majority of filament wound tanks utilize an extremely durable polyethylene liner.
Polyethylene, however, is not well suited for use with threaded fittings. Fittings attached to water treatment tanks must often be detached to allow for access to the tank interior for replenishment of the medium, to perform maintenance on the tank or fitting, or to exchange the fitting. The use of threaded unions to attach the tank liner, inlet, and outlet lines to these fittings is undesirable because the fittings and liner are often constructed of plastic and the cross-threading of plastic fittings is a significant problem. Such cross-threading occurs frequently when fittings are re-installed after removal and the resultant damage is not easily repaired. Plastic threads can also bind, making it extremely difficult to remove a previously installed fitting.
A two piece liner having a polyethylene vessel and a threaded insert with more durable threads has been developed. Conventional polyethylene tanks often have a two piece liner and a filament wound outer shell as disclosed in U.S. Pat. No. 3,874,544 and U.S. Pat. No. 4,579,242, the disclosures of which are explicitly incorporated by reference. The tank liner generally consists of an internally threaded insert and a vessel which is blow molded around the insert as disclosed in U.S. Pat. No. 4,589,563, the disclosure of which is explicitly incorporated by reference. An injection molded internally threaded tank end can also be bonded to an ABS tank liner after the liner has been blow molded.
All such two piece liners have a seam, glue joint or heat welded bond where the two pieces interface and such joints can contain voids and structural weaknesses. When pressurized, the joint between the insert and vessel is subjected to stresses and may form small cracks or the bond may fail which can create leakage problems.
Another method which has been developed to avoid the problems presented by plastic threads is to utilize a flanged tank insert for use with clamps and O-rings. Although this type of construction avoids the use of plastic threads it still includes the use of a tank insert. For example, a tank insert may have a flange, on top of which an O-ring is seated, followed by a basket filter flange, a second O-ring and then a valve flange. The flanges and O-rings are then all secured together by a four piece collar shaped clamp. One problem with this method is that the O-rings are disposed axially and the pressure of the contained fluid generates a force perpendicular to the bearing forces the sealed parts are exerting against the O-ring. As the pressure of the contained fluid is increased the sealed parts tend to pull apart axially. This axial force lessens the bearing force on the O-ring and thereby its effectiveness. The pressure forces also stress the joint between the tank vessel and insert which, as previously mentioned, can lead to potential leakage and structural problems.
Another problem with conventional tanks is the lack of a convenient port to access the tank interior. One option has been to provide an additional dome hole near the top of the tank. However, in conventional filament wound tanks, such dome holes can weaken the structural integrity of the tank, be expensive to produce and pose a potential leakage problem.
Female plastic threads present problems not only for the liner and fitting connection but also for connections involving the inlet and outlet lines. In response to this problem, a spring clip has been developed for coupling inlet and outlet lines to plastic fittings without the use of female threads. The inwardly directed tabs of a partially circular resilient spring clip are inserted into slots in the fitting to secure the inlet and outlet lines. The tabs extend through the slots and into an annular groove in an insert which has been affixed to the inlet or outlet line. The line is thereby removably secured to the fitting without a threaded connection between the fitting and the line. One problem with such a connection, however, is that when the line is not under pressure the clip may loosen and can be dislodged from the fitting.